The present invention relates to a foam control apparatus and is directed more particularly to a defoaming apparatus which optimizes the rate at which a defoaming agent is supplied to a processing vessel.
In many processes in which a solid is treated by exposure to a liquid, the formation of a foam can impair the efficiency or operability of the process. In the paper industry, for example, the formation of excessive quantities of foam in pulp washers and pulp thickeners can result in vessel overflows or in equipment clogging. In order to prevent this excessive foam formation, it is a common practice to introduce a defoaming agent into the processing vessel. Because the quantity of foam that forms varies from batch to batch in a batch process, or with the rate of stock flow in a continuous-flow process, it is often difficult to determine the quantity or rate at which defoaming agent should be added.
Some process operators have dealt with the above difficulty by using a quantity or rate of flow of defoaming agent which is large enough to accommodate any possible quantity of foam. This approach has proved expensive, however, because it necessitates the use of an excess of defoaming agent under most operating conditions.
Other process operators have dealt with the above difficulty by using a single foam level sensing probe and by introducing defoaming agent into the vessel only when the top of the form makes contact with the probe. In such systems, the addition of defoaming agent continues until the top of foam once again loses contact with the probe. This approach can also be costly, depending upon the depth of the liquid beneath the foam. If, for example, the vessel contains a large quantity of liquid, the probe will tend to cause a large quantity of defoaming agent to be added thereby producing an unnecessarily thin foam layer. If, on the other hand, the vessel contains only a small quantity of liquid, the probe will cause little or no defoaming agent to be added, thereby producing an excessively thick foam layer.